Circuits and methods for driving light sources

ABSTRACT

A controller for controlling dimming of a light-emitting diode (LED) light source provides a pulse signal by comparing a sensing signal indicative of a current flowing through the LED light source to a reference signal. The controller controls the current through the LED light source according to the pulse signal during a first state of a pulse-width modulation signal and cutting off the current through the LED light source during a second state of the pulse-width modulation signal. The controller receives a dimming request signal indicative of an operation of a power switch coupled between an AC power source and a bridge rectifier and adjusts both a level of the reference signal and a duty cycle of the pulse-width modulation signal based on the dimming request signal.

RELATED APPLICATION

This application is a continuation-in-part of the co-pending U.S. patentapplication Ser. No. 12/415,028, entitled “Driving Circuit withContinuous Dimming Function for Driving Light sources”, filed on Mar.31, 2009, which itself is a continuation-in-part of the co-pending U.S.patent application Ser. No. 12/316,480, filed on Dec. 12, 2008, entitled“Driving Circuit with Dimming Controller for Driving Light Sources” andboth of which are fully incorporated herein by reference.

BACKGROUND

In recent years, light sources such as light-emitting diodes (LEDs) havebeen improved through technological advances in material andmanufacturing processes. LED possesses relatively high efficiency, longlife, vivid colors and can be used in a variety of industries includingthe automotive, computer, telecom, military and consumer goods, etc. Oneexample is an LED lamp which uses LEDs to replace traditional lightsources such as electrical filament.

FIG. 1 shows a schematic diagram of a conventional LED driving circuit100. The LED driving circuit 100 utilizes an LED string 106 as a lightsource. The LED string 106 includes a group of LEDs connected in series.A power converter 102 converts an input voltage Vin to a desired outputDC voltage Vout for powering the LED string 106. A switch 104 coupled tothe LED driving circuit 100 can enable or disable the input voltage Vinto the LED string 106, and therefore can turn on or turn off the LEDlamp. The power converter 102 receives a feedback signal from a currentsensing resistor Rsen and adjusts the output voltage Vout to make theLED string 106 generate a desired light output. One of the drawbacks ofthis solution is that a desired light output is predetermined. Inoperation, the light output of the LED string 106 is set to apredetermined level and may not be adjusted by users.

FIG. 2 illustrates a schematic diagram of another conventional LEDdriving circuit 200. A power converter 102 converts an input voltage Vinto a desired output DC voltage Vout for powering the LED string 106. Aswitch 104 coupled to LED driving circuit 100 can enable or disable theinput voltage Vin to the LED string 106, and therefore can turn on orturn off the LED lamp. The LED string 106 is coupled to a linear LEDcurrent regulator 208. Operational amplifiers 210 in the linear LEDcurrent regulator 208 compares a reference signal REF and a currentmonitoring signal from current sensing resistor Rsen, and generates acontrol signal to adjust the resistance of transistor Q1 in a linearmode. Therefore, the LED current flowing through the LED string 106 canbe adjusted accordingly. In this solution, in order to control the lightoutput of the LED string 106, users may need to use a dedicatedapparatus, such as a specially designed switch with adjusting buttons ora switch that can receive a remote control signal, to adjust thereference signal REF.

SUMMARY

In one embodiment, a controller for controlling dimming of alight-emitting diode (LED) light source provides a pulse signal bycomparing a sensing signal indicative of a current flowing through theLED light source to a reference signal. The controller controls thecurrent through the LED light source according to the pulse signalduring a first state of a pulse-width modulation signal and cutting offthe current through the LED light source during a second state of thepulse-width modulation signal. The controller receives a dimming requestsignal indicative of an operation of a power switch coupled between anAC power source and a bridge rectifier and adjusts both a level of thereference signal and a duty cycle of the pulse-width modulation signalbased on the dimming request signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matterwill become apparent as the following detailed description proceeds, andupon reference to the drawings, wherein like numerals depict like parts,and in which:

FIG. 1 shows a schematic diagram of a conventional LED driving circuit.

FIG. 2 shows a schematic diagram of another conventional LED drivingcircuit.

FIG. 3 shows a block diagram of a light source driving circuit, inaccordance with one embodiment of the present invention.

FIG. 4 shows a schematic diagram of a light source driving circuit, inaccordance with one embodiment of the present invention.

FIG. 5 shows a structure of a dimming controller in FIG. 4, inaccordance with one embodiment of the present invention.

FIG. 6 illustrates signal waveforms in the analog dimming mode, inaccordance with one embodiment of the present invention.

FIG. 7 illustrates signal waveforms in the burst dimming mode, inaccordance with one embodiment of the present invention.

FIG. 8 illustrates a diagram illustrating an operation of a light sourcedriving circuit which includes the dimming controller in FIG. 5, inaccordance with one embodiment of the present invention.

FIG. 9 shows a flowchart of a method for adjusting power of a lightsource, in accordance with one embodiment of the present invention.

FIG. 10 shows a schematic diagram of a light source driving circuit, inaccordance with one embodiment of the present invention.

FIG. 11 shows a structure of a dimming controller in FIG. 10, inaccordance with one embodiment of the present invention.

FIG. 12 illustrates a diagram illustrating an operation of a lightsource driving circuit which includes the dimming controller in FIG. 11,in accordance with one embodiment of the present invention.

FIG. 13 shows a flowchart of a method for adjusting power of a lightsource, in accordance with one embodiment of the present invention.

FIG. 14A shows an example of a schematic diagram of a light sourcedriving circuit, in accordance with one embodiment of the presentinvention.

FIG. 14B shows an example of a power switch in FIG. 14A, in accordancewith one embodiment of the present invention.

FIG. 15 shows an example of a structure of a dimming controller in FIG.14, in accordance with one embodiment of the present invention.

FIG. 16 illustrates an example of a diagram illustrating an operation ofa light source driving circuit which includes the dimming controller inFIG. 15, in accordance with one embodiment of the present invention.

FIG. 17 illustrates another example of a diagram illustrating anoperation of a light source driving circuit which includes the dimmingcontroller in FIG. 15, in accordance with one embodiment of the presentinvention.

FIG. 18 shows a flowchart of a method for adjusting power of a lightsource, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the presentinvention. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims.

Furthermore, in the following detailed description of the presentinvention, numerous specific details are set forth in order to provide athorough understanding of the present invention. However, it will berecognized by one of ordinary skill in the art that the presentinvention may be practiced without these specific details. In otherinstances, well known methods, procedures, components, and circuits havenot been described in detail as not to unnecessarily obscure aspects ofthe present invention.

FIG. 3 shows an example of a block diagram of a light source drivingcircuit 300, in accordance with one embodiment of the present invention.In one embodiment, a power switch 304 coupled between a power source Vinand the light source driving circuit 300 is operable for selectivelycoupling the power source to the light source driving circuit 300. Thelight source driving circuit 300 includes an AC/DC converter 306 forconverting an AC input voltage Vin from the power source to a DC voltageVout, a power converter 310 coupled to the AC/DC converter 306 forproviding an LED string 312 with a regulated power, a dimming controller308 coupled to the power converter 310 for receiving a switch monitoringsignal indicative of an operation of the power switch 304 and foradjusting the regulated power from the power converter 310 according tothe switch monitoring signal, and a current sensor 314 for sensing anLED current flowing through the LED string 312. In one embodiment, thepower switch 304 can be an on/off switch mounted on the wall.

In operation, the AC/DC converter 306 converts the input AC voltage Vinto the output DC voltage Vout. The power converter 310 receives the DCvoltage Vout and provides the LED string 312 with a regulated power. Thecurrent sensor 314 generates a current monitoring signal indicating alevel of an LED current flowing through the LED string 312. The dimmingcontroller 308 monitors the operation of the power switch 304, receivesthe current monitoring signal from the current sensor 314, and isoperable for controlling the power converter 310 to adjust power of theLED string 312 in response to the operation of the power switch 304. Inone embodiment, the dimming controller 308 operates in an analog dimmingmode and adjusts the power of the LED string 312 by adjusting areference signal indicating a peak value of the LED current. In anotherembodiment, the dimming controller 308 operates in a burst dimming modeand adjusts the power of the LED string 312 by adjusting a duty cycle ofa pulse width modulation (PWM) signal. By adjusting the power of the LEDstring 312, the light output of the LED string 312 can be adjustedaccordingly.

FIG. 4 shows an example of a schematic diagram of a light source drivingcircuit 400, in accordance with one embodiment of the present invention.FIG. 4 is described in combination with FIG. 3. Elements labeled thesame as in FIG. 3 have similar functions and will not be detaileddescribed herein.

The light source driving circuit 400 includes a power converter 310(shown in FIG. 3) coupled to a power source and coupled to an LED string312 for receiving power from the power source and for providing aregulated power to the LED string 312. In the example of FIG. 4, thepower converter 310 can be a buck converter including an inductor L1, adiode D4 and a control switch Q16. In the embodiment shown in FIG. 4,the control switch Q16 is implemented outside the dimming controller308. In another embodiment, the control switch Q16 can be integrated inthe dimming controller 308.

A dimming controller 308 is operable for receiving a switch monitoringsignal indicative of an operation of a power switch, e.g., a powerswitch 304 coupled between the power source Vin and the light sourcedriving circuit 400, and for adjusting the regulated power from thepower converter 310 (including the inductor L1, the diode D4 and thecontrol switch Q16) by controlling the control switch Q16 coupled inseries with the LED string 312 according to the switch monitoringsignal. The light source driving circuit 400 can further include anAC/DC converter 306 for converting an AC input voltage Vin to a DCoutput voltage Vout, and a current sensor 314 for sensing an LED currentflowing through the LED string 312. In the example of FIG. 4, the AC/DCconverter 306 can include a bridge rectifier including diodes D1, D2, D7and D8. The current sensor 314 can include a current sensing resistorR5.

In one embodiment, terminals of the dimming controller 308 can includeHV_GATE, SEL, CLK, RT, VDD, CTRL, MON and GND. The terminal HV_GATE iscoupled to a switch Q27 through a resistor R15 for controlling aconductance status, e.g., ON/OFF status, of the switch Q27 coupled tothe LED string 312. A capacitor C11 is coupled between the terminalHV_GATE and ground for regulating a gate voltage of the switch Q27.

A user can select a dimming mode, e.g., an analog dimming mode or aburst dimming mode, by coupling the terminal SEL to ground through aresistor R4 (as shown in FIG. 4), or coupling the terminal SEL to grounddirectly.

The terminal CLK is coupled to the AC/DC converter 306 through aresistor R3, and is coupled to ground through a resistor R6. Theterminal CLK can receive a switch monitoring signal indicating anoperation of the power switch 304. In one embodiment, the switchmonitoring signal can be generated at a common node between the resistorR3 and the resistor R6. A capacitor C12 is coupled to the resistor R6 inparallel for filtering undesired noises. The terminal RT is coupled toground through a resistor R7 for determining a frequency of a pulsesignal generated by the dimming controller 308.

The terminal VDD is coupled to the switch Q27 through a diode D9 forsupplying power to the dimming controller 308. In one embodiment, anenergy storage unit, e.g., a capacitor C10, coupled between the terminalVDD and ground can power the dimming controller 308 when the powerswitch 304 is turned off. In an alternate embodiment, the energy storageunit can be integrated in the dimming controller 308. The terminal GNDis coupled to ground.

The terminal CTRL is coupled to the control switch Q16. The controlswitch Q16 is coupled in series with the LED string 312 and the switchQ27, and is coupled to ground through the current sensing resistor R5.The dimming controller 308 is operable for adjusting the regulated powerfrom the power converter 310 by controlling a conductance status, e.g.,ON and OFF status, of the control switch Q16 using a control signal viathe terminal CTRL. The terminal MON is coupled to the current sensingresistor R5 for receiving a current monitoring signal indicating an LEDcurrent flowing through the LED string 312. When the switch Q27 isturned on, the dimming controller 308 can adjust the LED current flowingthrough the LED string 312 to ground by controlling the control switchQ16.

In operation, when the power switch 304 is turned on, the AC/DCconverter 306 converts an input AC voltage Vin to a DC voltage Vout. Apredetermined voltage at the terminal HV_GATE is supplied to the switchQ27 through the resistor R15 so that the switch Q27 is turned on.

If the dimming controller 308 turns on the control switch Q16, the DCvoltage Vout powers the LED string 312 and charges the inductor L1. AnLED current flows through the inductor L1, the LED string 312, theswitch Q27, the control switch Q16, the current sensing resistor R5 toground. If the dimming controller 308 turns off the control switch Q16,an LED current flows through the inductor L1, the LED string 312 and thediode D4. The inductor L1 is discharged to power the LED string 312. Assuch, the dimming controller 308 can adjust the regulated power from thepower converter 310 by controlling the control switch Q16.

When the power switch 304 is turned off, the capacitor C10 is dischargedto power the dimming controller 308. A voltage across the resistor R6drops to zero, therefore a switch monitoring signal indicating aturn-off operation of the power switch 304 can be detected by thedimming controller 308 through the terminal CLK. Similarly, when thepower switch 304 is turned on, the voltage across the resistor R6 risesto a predetermined voltage, therefore a switch monitoring signalindicating a turn-on operation of the power switch 304 can be detectedby the dimming controller 308 through the terminal CLK. If a turn-offoperation is detected, the dimming controller 308 can turn off theswitch Q27 by pulling the voltage at the terminal HV_GATE to zero suchthat the LED string 312 can be turned off after the inductor L1completes discharging. In response to the turn-off operation, thedimming controller 308 can adjust a reference signal indicating a targetlight output of the LED string 312. Therefore, when the power switch 304is turned on next time, the LED string 312 can generate a light outputaccording to the adjusted target light output. In other words, the lightoutput of the LED string 312 can be adjusted by the dimming controller308 in response to the turn-off operation of the power switch 304.

FIG. 5 shows an example of a structure of the dimming controller 308 inFIG. 4, in accordance with one embodiment of the present invention. FIG.5 is described in combination with FIG. 4. Elements labeled the same asin FIG. 4 have similar functions and will not be detailed describedherein.

The dimming controller 308 includes a trigger monitoring unit 506, adimmer 502 and a pulse signal generator 504. The trigger monitoring unit506 is coupled to ground through a Zener diode ZD1. The triggermonitoring unit 506 can receive a switch monitoring signal indicating anoperation of the external power switch 304 through the terminal CLK andcan generate a driving signal for driving a counter 526 when anoperation of the external power switch 304 is detected at the terminalCLK. The trigger monitoring unit 506 is further operable for controllinga conductance status of the switch Q27. The dimmer 502 is operable forgenerating a reference signal REF to adjust power of the LED string 312in an analog dimming mode, or generating a control signal 538 foradjusting a duty cycle of a PWM signal PWM1 to adjust the power of theLED string 312. The pulse signal generator 504 is operable forgenerating a pulse signal which can turn on a control switch Q16. Thedimming controller 308 can further include a start up and under voltagelockout (UVL) circuit 508 coupled to the terminal VDD for selectivelyturning on one or more components of the dimming controller 308according to different power condition.

In one embodiment, the start up and under voltage lockout circuit 508 isoperable for turning on all the components of the dimming controller 308when the voltage at the terminal VDD is greater than a firstpredetermined voltage. When the power switch 304 is turned off, thestart up and under voltage lockout circuit 508 is operable for turningoff other components of the dimming controller 308 except the triggermonitoring unit 506 and the dimmer 502 when the voltage at the terminalVDD is less than a second predetermined voltage, in order to saveenergy. The start up and under voltage lockout circuit 508 is furtheroperable for turning off the trigger monitoring unit 506 and the dimmer502 when the voltage at the terminal VDD is less than a thirdpredetermined voltage. In one embodiment, the first predeterminedvoltage is greater than the second predetermined voltage and the secondpredetermined voltage is greater than the third predetermined voltage.Because the dimming controller 308 can be powered by the capacitor C10through the terminal VDD, the trigger monitoring unit 506 and the dimmer502 can still operate for a time period after the power switch 304 isturned off.

In the dimming controller 308, the terminal SEL is coupled to a currentsource 532. Users can choose a dimming mode by configuring the terminalSEL, e.g., by coupling the terminal SEL directly to ground or couplingthe terminal SEL to ground via a resistor. In one embodiment, thedimming mode can be determined by measuring a voltage at the terminalSEL. If the terminal SEL is directly coupled to ground, the voltage atthe terminal SEL is approximately equal to zero. A control circuit canin turn switch on a switch 540, switch off a switch 541 and switch off aswitch 542. Therefore, the dimming controller 308 can work in an analogdimming mode and can adjust the power of the LED string 312 (shown inFIG. 4) by adjusting a reference signal REF. In one embodiment, if theterminal SEL is coupled to ground via a resistor R4 having apredetermined resistance (as shown in FIG. 4), the voltage at theterminal SEL can be greater than zero. The control circuit can in turnswitch off the switch 540, switch on the switch 541 and switch on theswitch 542. Therefore, the dimming controller 308 can work in a burstdimming mode and can adjust the power of the LED string 312 (shown inFIG. 4) by adjusting a duty cycle of a PWM signal PWM1. In other words,different dimming modes can be selected by controlling the ON/OFF statusof the switch 540, switch 541 and switch 542. The ON/OFF status of theswitch 540, switch 541 and switch 542 can be determined by the voltageat the terminal SEL.

The pulse signal generator 504 is coupled to ground through the terminalRT and the resistor R7 for generating a pulse signal 536 which can turnon the control switch Q16. The pulse signal generator 504 can havedifferent configurations and is not limited to the configuration asshown in the example of FIG. 5.

In the pulse signal generator 504, the non-inverting input of anoperational amplifier 510 receives a predetermined voltage V1. Thus, thevoltage of the inverting input of the operational amplifier 510 can beforced to V1. A current IRT flows through the terminal RT and theresistor R7 to ground. A current I1 flowing through a MOSFET 514 and aMOSFET 515 is equal to IRT. Because the MOSFET 514 and a MOSFET 512constitute a current mirror, a current I2 flowing through the MOSFET 512is also substantially equal to IRT. The output of a comparator 516 andthe output of a comparator 518 are respectively coupled to the S inputand the R input of an SR flip-flop 520. The inverting input of thecomparator 516 receives a predetermined voltage V2. The non-invertinginput of the comparator 518 receives a predetermined voltage V3. V2 isgreater than V3, and V3 is greater than zero, in one embodiment. Acapacitor C4 is coupled between the MOSFET 512 and ground, and has oneend coupled to a common node between the non-inverting input of thecomparator 516 and the inverting input of the comparator 518. The Qoutput of the SR flip-flop 520 is coupled to the switch Q15 and the Sinput of an SR flip-flop 522. The switch Q15 is coupled in parallel withthe capacitor C4. A conductance status, e.g., ON/OFF status, of theswitch Q15 can be determined by the Q output of the SR flip-flop 520.

Initially, the voltage across the capacitor C4 is approximately equal tozero which is less than V3. Therefore, the R input of the SR flip-flop520 receives a digital 1 from the output of the comparator 518. The Qoutput of the SR flip-flop 520 is set to digital 0, which turns off theswitch Q15. When the switch Q15 is turned off, the voltage across thecapacitor C4 increases as the capacitor C4 is charged by I2. When thevoltage across C4 is greater than V2, the S input of the SR flip-flop520 receives a digital 1 from the output of the comparator 516. The Qoutput of the SR flip-flop 520 is set to digital 1, which turns on theswitch Q15. When the switch Q15 is turned on, the voltage across thecapacitor C4 decreases as the capacitor C4 discharges through the switchQ15. When the voltage across the capacitor C4 drops below V3, thecomparator 518 outputs a digital 1, and the Q output of the SR flip-flop520 is set to digital 0, which turns off the switch Q15. Then thecapacitor C4 is charged by I2 again. As such, through the processdescribed above, the pulse signal generator 504 can generate a pulsesignal 536 which includes a series of pulses at the Q output of the SRflip-flop 520. The pulse signal 536 is sent to the S input of the SRflip-flop 522.

The trigger monitoring unit 506 is operable for monitoring an operationof the power switch 304 through the terminal CLK, and is operable forgenerating a driving signal for driving the counter 526 when anoperation of the power switch 304 is detected at the terminal CLK. Inone embodiment, when the power switch 304 is turned on, the voltage atthe terminal CLK rises to a level that is equal to a voltage across theresistor R6 (shown in FIG. 4). When the power switch 304 is turned off,the voltage at the terminal CLK drops to zero. Therefore, a switchmonitoring signal indicating the operation of the power switch 304 canbe detected at the terminal CLK. In one embodiment, the triggermonitoring unit 506 generates a driving signal when a turn-off operationis detected at the terminal CLK.

The trigger monitoring unit 506 is further operable for controlling aconductance status of the switch Q27 through the terminal HV_GATE. Whenthe power switch 304 is turned on, a breakdown voltage across the Zenerdiode ZD1 is applied to the switch Q27 through the resistor R3.Therefore, the switch Q27 can be turned on. The trigger monitoring unit506 can turn off the switch Q27 by pulling the voltage at the terminalHV_GATE to zero. In one embodiment, the trigger monitoring unit 506turns off the switch Q27 when a turn-off operation of the power switch304 is detected at the terminal CLK and turns on the switch Q27 when aturn-on operation of the power switch 304 is detected at the terminalCLK.

In one embodiment, the dimmer 502 includes a counter 526 coupled to thetrigger monitoring unit 506 for counting operations of the power switch304, a digital-to-analog converter (D/A converter) 528 coupled to thecounter 526. The dimmer 502 can further include a PWM generator 530coupled to the D/A converter 528. The counter 526 can be driven by thedriving signal generated by the trigger monitoring unit 506. Morespecifically, when the power switch 304 is turned off, the triggermonitoring unit 506 detects a negative edge of the voltage at theterminal CLK and generates a driving signal, in one embodiment. Thecounter value of the counter 526 can be increased, e.g., by 1, inresponse to the driving signal. The D/A converter 528 reads the countervalue from the counter 526 and generates a dimming signal (e.g., controlsignal 538 or reference signal REF) based on the counter value. Thedimming signal can be used to adjust a target power level of the powerconverter 310, which can in turn adjust the light output of the LEDstring 312.

In the burst dimming mode, the switch 540 is off, the switch 541 and theswitch 542 are on. The inverting input of the comparator 534 receives areference signal REF1 which can be a DC signal having a predeterminedsubstantially constant voltage. The voltage of REF1 can determine a peakvalue of the LED current, which can in turn determine the maximum lightoutput of the LED string 312. The dimming signal can be a control signal538 which is applied to the PWM generator 530 for adjusting a duty cycleof the PWM signal PWM1. By adjusting the duty cycle of PWM1, the lightoutput of the LED string 312 can be adjusted no greater than the maximumlight output determined by REF1. For example, if PWM1 has a duty cycleof 100%, the LED string 312 can have the maximum light output. If theduty cycle of PWM1 is less than 100%, the LED string 312 can have alight output that is lower than the maximum light output.

In the analog dimming mode, the switch 540 is on, the switch 541 and theswitch 542 are off, and the dimming signal can be an analog referencesignal REF having an adjustable voltage. The D/A converter 528 canadjust the voltage of the reference signal REF according to the countervalue of the counter 526. The voltage of REF can determine a peak valueof the LED current, which can in turn determine an average value of theLED current. As such, the light output of the LED string 312 can beadjusted by adjusting the reference signal REF.

In one embodiment, the D/A converter 528 can decrease the voltage of REFin response to an increase of the counter value. For example, if thecounter value is 0, the D/A converter 528 adjusts the reference signalREF to have a voltage V4. If the counter value is increased to 1 when aturn-off operation of the power switch 304 is detected at the terminalCLK by the trigger monitoring unit 506, the D/A converter 528 adjuststhe reference signal REF to have a voltage V5 that is less than V4. Yetin another embodiment, the D/A converter 528 can increase the voltage ofREF in response to an increase of the counter value.

In one embodiment, the counter value will be reset to zero after thecounter 526 reaches its maximum counter value. For example, if thecounter 526 is a 2-bit counter, the counter value will increase from 0to 1, 2, 3 and then return to zero after four turn-off operations havebeen detected. Accordingly, the light output of the LED string 312 canbe adjusted from a first level to a second level, then to a third level,then to a fourth level, and then back to the first level.

The inverting input of a comparator 534 can selectively receive thereference signal REF and the reference signal REF1. For example, theinverting input of the comparator 534 receives the reference signal REFthrough the switch 540 in the analog dimming mode, and receives thereference signal REF1 through the switch 541 in the burst dimming mode.The non-inverting input of the comparator 534 is coupled to the resistorR5 through the terminal MON for receiving a current monitoring signalSEN from the current sensing resistor R5. The voltage of the currentmonitoring signal SEN can indicate an LED current flowing through theLED string 312 when the switch Q27 and the control switch Q16 are turnedon.

The output of the comparator 534 is coupled to the R input of the SRflip-flop 522. The Q output of the SR flip-flop 522 is coupled to an ANDgate 524. The PWM signal PWM1 generated by the PWM generator 530 isapplied to the AND gate 524. The AND gate 524 outputs a control signalto control the control switch Q16 through the terminal CTRL.

If the analog dimming mode is selected, the switch 540 is turned on andthe switches 541 and 542 are turned off. The control switch Q16 iscontrolled by the SR flip-flop 522. In operation, when the power switch304 is turned on, the breakdown voltage across the Zener diode ZD1 turnson the switch Q27. The SR flip-flop 522 generates a digital 1 at the Qoutput to turn on the control switch Q16 in response to the pulse signal536 generated by the pulse generator 504. An LED current flowing throughthe inductor L1, the LED string 312, the switch Q27, the control switchQ16, the current sensing resistor R5 to ground. The LED currentgradually increases because the inductor resists a sudden change of theLED current. As a result, the voltage across the current sensingresistor R5, that is, the voltage of the current monitoring signal SENcan be increased. When the voltage of SEN is greater than that of thereference signal REF, the comparator 534 generates a digital 1 at the Rinput of the SR flip-flop 522 so that the SR flip-flop 522 generates adigital 0 to turn off the control switch Q16. After the control switchQ16 is turned off, the inductor L1 is discharged to power the LED string312. An LED current which flows through the inductor L1, the LED string312 and the diode D4 gradually decreases. The control switch Q16 isturned on when the SR flip-flop 522 receives a pulse at the S inputagain, and then the LED current flows through the current sensingresistor R5 to ground again. When the voltage of the current monitoringsignal SEN is greater than that of the reference signal REF, the controlswitch Q16 is turned off by the SR flip-flop 522. As described above,the reference signal REF determines a peak value of the LED current,which can in turn determine the light output of the LED string 312. Byadjusting the reference signal REF, the light output of the LED string312 can be adjusted.

In the analog dimming mode, when the power switch 304 is turned off, thecapacitor C10 (shown in FIG. 4) is discharged to power the dimmingcontroller 308. The counter value of the counter 526 can be increased by1 when the trigger monitoring unit 506 detects a turn-off operation ofthe power switch 304 at the terminal CLK. The trigger monitoring unit506 can turn off the switch Q27 in response to the turn-off operation ofthe power switch 304. The D/A converter 528 can adjust the voltage ofthe reference signal REF from a first level to a second level inresponse to the change of the counter value. Therefore, the light outputof the LED string 312 can be adjusted in accordance with the adjustedreference signal REF when the power switch 304 is turned on.

If the burst dimming mode is selected, the switch 540 is turned off andthe switches 541 and 542 are turned on. The inverting input of thecomparator 534 receives a reference signal REF1 having a predeterminedvoltage. The control switch Q16 is controlled by both of the SRflip-flop 522 and the PWM signal PWM1 through the AND gate 524. Thereference signal REF1 can determine a peak value of the LED current,which can in turn determine a maximum light output of the LED string312. The duty cycle of the PWM signal PWM1 can determine the on/off timeof the control switch Q16. When the PWM signal PWM1 is logic 1, theconductance status of the control switch Q16 is determined by the Qoutput of the SR flip-flop 522. When the PWM signal PWM1 is logic 0, thecontrol switch Q16 is turned off. By adjusting the duty cycle of the PWMsignal PWM1, the power of the LED string 312 can be adjustedaccordingly. As such, the combination of the reference signal REF1 andthe PWM signal PWM1 can determine the light output of the LED string312.

In the burst dimming mode, when the power switch 304 is turned off, aturn-off operation of the power switch 304 can be detected by thetrigger monitoring unit 506 at the terminal CLK. The trigger monitoringunit 506 turns off the switch Q27 and generates a driving signal. Thecounter value of the counter 526 can be increased, e.g., by 1, inresponse of the driving signal. The D/A converter 528 can generate thecontrol signal 538 to adjust the duty cycle of the PWM signal PWM1 froma first level to a second level. Therefore, when the power switch 304 isturned on next time, the light output of the LED string 312 can beadjusted to follow a target light output which is determined by thereference signal REF1 and the PWM signal PWM1.

FIG. 6 illustrates examples of signal waveforms of an LED current 602flowing through the LED string 312, the pulse signal 536, V522 whichindicates the output of the SR flip-flop 522, V524 which indicates theoutput of the AND gate 524, and the ON/OFF status of the control switchQ16 in the analog dimming mode. FIG. 6 is described in combination withFIG. 4 and FIG. 5.

In operation, the pulse signal generator 504 generates pulse signal 536.The SR flip-flop 522 generates a digital 1 at the Q output in responseto each pulse of the pulse signal 536. The control switch Q16 is turnedon when the Q output of the SR flip-flop 522 is digital 1. When thecontrol switch Q16 is turned on, the inductor L1 ramps up and the LEDcurrent 602 increases. When the LED current 602 reaches the peak valueImax, which means the voltage of the current monitoring signal SEN issubstantially equal to the voltage of the reference signal REF, thecomparator 534 generates a digital 1 at the R input of the SR flip-flop522 so that the SR flip-flop 522 generates a digital 0 at the Q output.The control switch Q16 is turned off when the Q output of the SRflip-flop 522 is digital 0. When the control switch Q16 is turned off,the inductor L1 is discharged to power the LED string 312 and the LEDcurrent 602 decreases. In this analog dimming mode, by adjusting thereference signal REF, the average LED current can be adjustedaccordingly and therefore the light output of the LED string 312 can beadjusted.

FIG. 7 illustrates examples of signal waveforms of the LED current 602flowing through the LED string 312, the pulse signal 536, V522 whichindicates the output of the SR flip-flop 522, V524 which indicates theoutput of the AND gate 524, and the ON/OFF status of the control switchQ16, and the PMW signal PWM1 in the burst dimming mode. FIG. 7 isdescribed in combination with FIG. 4 and FIG. 5.

When PWM1 is digital 1, the relationship among the LED current 602, thepulse signal 536, V522, V524, and the ON/OFF status of the switch Q1 issimilar to that is illustrated in FIG. 6. When PWM1 is digital 0, theoutput of the AND gate 524 turns to digital 0. Therefore, the controlswitch Q16 is turned off and the LED current 602 decreases. If the PWM1holds digital 0 long enough, the LED current 602 can falls to zero. Inthis burst dimming mode, by adjusting the duty cycle of PWM1, theaverage LED current can be adjusted accordingly and therefore the lightoutput of the LED string 312 can be adjusted.

FIG. 8 shows an example of a diagram illustrating an operation of alight source driving circuit which includes the dimming controller inFIG. 5, in accordance with one embodiment of the present invention. FIG.8 is described in combination with FIG. 5.

In the example shown in FIG. 8, each time when a turn-off operation ofthe power switch 304 is detected by the trigger monitoring unit 506, thecounter value of the counter 526 is increases by 1. The counter 526 canbe a 2-bit counter which has a maximum counter value of 3.

In the analog dimming mode, the D/A converter 528 reads the countervalue from the counter 526 and decreases the voltage of the referencesignal REF in response to an increase of the counter value. The voltageof REF can determine a peak value Imax of the LED current, which can inturn determine an average value of the LED current. In the burst dimmingmode, the D/A converter 528 reads the counter value from the counter 526and decreases the duty cycle of the PWM signal PWM1 (e.g., decreases 25%each time) in response to an increase of the counter value. The counter526 is reset after it reaches its maximum counter value (e.g., 3).

FIG. 9 shows a flowchart 900 of a method for adjusting power of a lightsource, in accordance with one embodiment of the present invention. FIG.9 is described in combination with FIG. 4 and FIG. 5.

In block 902, a light source, e.g., the LED string 312, is powered by aregulated power from a power converter, e.g., the power converter 310.In block 904, a switch monitoring signal can be received, e.g., by thedimming controller 308. The switch monitoring signal can indicate anoperation of a power switch, e.g., the power switch 304 coupled betweena power source and the power converter. In block 906, a dimming signalis generated according to the switch monitoring signal. In block 908, aswitch coupled in series with the light source, e.g., the control switchQ16, is controlled according to the dimming signal so as to adjust theregulated power from the power converter. In one embodiment, in ananalog dimming mode, the regulated power from the power converter can beadjusted by comparing the dimming signal with a feedback currentmonitoring signal which indicates a light source current of the lightsource. In another embodiment, in a burst dimming mode, the regulatedpower from the power converter can be adjusted by controlling a dutycycle of a PWM signal by the dimming signal.

Accordingly, embodiments in accordance with the present inventionprovide a light source driving circuit that can adjust power of a lightsource according to a switch monitoring signal indicative of anoperation of a power switch, e.g., an on/off switch mounted on the wall.The power of the light source, which is provided by a power converter,can be adjusted by a dimming controller by controlling a switch coupledin series with the light source. Advantageously, as described above,users can adjust the light output of the light source through anoperation (e.g., a turn-off operation) of a common on/off power switch.Therefore, extra apparatus for dimming, such as an external dimmer or aspecially designed switch with adjusting buttons, can be avoided and thecost can be reduced.

FIG. 10 shows an example of a schematic diagram of a light sourcedriving circuit 1000, in accordance with one embodiment of the presentinvention. FIG. 10 is described in combination with FIG. 3. Elementslabeled the same as in FIG. 3 and FIG. 4 have similar functions.

The light source driving circuit 1000 includes a power converter 310coupled to a power source and an LED string 312 for receiving power fromthe power source and for providing a regulated power to the LED string312. A dimming controller 1008 is operable for monitoring a power switch304 coupled between the power source and the light source drivingcircuit 1000 by monitoring the voltage at a terminal CLK. The dimmingcontroller 1008 is operable for receiving a dimming request signalindicative of a first set of operations of the power switch 304 and forreceiving a dimming termination signal indicative of a second set ofoperations of the power switch 304. The dimming controller 1008 canreceive the dimming request signal and the dimming termination signalvia the terminal CLK. The dimming controller 1008 is further operablefor continuously adjusting the regulated power from the power converter310 if the dimming request signal is received, and for stoppingadjusting the regulated power from the power converter 310 if thedimming termination signal is received. In other words, the dimmingcontroller 1008 can continuously adjust the power from the powerconverter 310 upon detection of the first set of operations of the powerswitch 304 until the second set of operations of the power switch 304are detected. In one embodiment, the dimming controller 1008 can adjustthe regulated power from the power converter 310 by controlling acontrol switch Q16 coupled in series with the LED string 312.

FIG. 11 shows an example of a structure of the dimming controller 1008in FIG. 10, in accordance with one embodiment of the present invention.FIG. 11 is described in combination with FIG. 10. Elements labeled thesame as in FIG. 4, FIG. 5 and FIG. 10 have similar functions.

In the example of FIG. 11, the structure of the dimming controller 1008in FIG. 11 is similar to the structure of the dimming controller 308 inFIG. 5 except for the configuration of the dimmer 1102 and the triggermonitoring unit 1106. In FIG. 11, the trigger monitoring unit 1106 isoperable for receiving the dimming request signal and the dimmingtermination signal via the terminal CLK, and for generating a signal ENto enable or disable a clock generator 1104. The trigger monitoring unit1106 is further operable for controlling a conductance status of theswitch Q27 coupled to the LED string 312.

The dimmer 1102 is operable for generating a reference signal REF toadjust power of the LED string 312 in an analog dimming mode, orgenerating a control signal 538 for adjusting a duty cycle of a PWMsignal PWM1 to adjust the power of the LED string 312 in a burst dimmingmode. In the example shown in FIG. 11, the dimmer 1102 can include theclock generator 1104 coupled to the trigger monitoring unit 1106 forgenerating a clock signal, a counter 1126 driven by the clock signal, andigital-to-analog (D/A) converter 528 coupled to the counter 1126. Thedimmer 1102 can further include a PWM generator 530 coupled to the D/Aconverter 528.

In operation, when the power switch 304 is turned on or turned off, thetrigger monitoring unit 1106 can detect a positive edge or a negativeedge of the voltage at the terminal CLK. For example, when the powerswitch 304 is turned off, the capacitor C10 is discharged to power thedimming controller 1108. A voltage across the resistor R6 drops to zero.Therefore, a negative edge of the voltage at the terminal CLK can bedetected by the trigger monitoring unit 1106. Similarly, when the powerswitch 304 is turned on, the voltage across the resistor R6 rises to apredetermined voltage. Therefore, a positive edge of the voltage at theterminal CLK can be detected by the trigger monitoring unit 1106. Assuch, operations, e.g., turn-on operations or turn-off operations, ofthe power switch 304 can be detected by the trigger monitoring unit 1106by monitoring the voltage at the terminal CLK.

In one embodiment, a dimming request signal can be received by thetrigger monitoring unit 1106 via the terminal CLK when a first set ofoperations of the power switch 304 are detected. A dimming terminationsignal can be received by the trigger monitoring unit 1106 via theterminal CLK when a second set of operations of the power switch 304 aredetected. In one embodiment, the first set of operations of the powerswitch 304 includes a first turn-off operation followed by a firstturn-on operation. In one embodiment, the second set of operations ofthe power switch 304 includes a second turn-off operation followed by asecond turn-on operation.

If the dimming request signal is received by the trigger monitoring unit1106, the dimming controller 1108 begins to continuously adjust theregulated power from the power converter 310. In an analog dimming mode,the dimming controller 1108 adjusts a voltage of a reference signal REFto adjust the regulated power from the power converter 310. In a burstdimming mode, the dimming controller 1108 adjusts a duty cycle of a PWMsignal PWM1 to adjust the regulated power from the power converter 310.

If the dimming termination signal is received by the trigger monitoringunit 1106, the dimming controller 1108 can stop adjusting the regulatedpower from the power converter 310.

FIG. 12 illustrates an example of a diagram illustrating an operation ofa light source driving circuit which includes the dimming controller1008 in FIG. 11, in accordance with one embodiment of the presentinvention. FIG. 12 is described in combination with FIG. 10 and FIG. 11.

Assume that initially the power switch 304 is off. In operation, whenthe power switch 304 is turned on, e.g., by a user, the LED string 312is powered by a regulated power from the power converter 310 to generatean initial light output, in one embodiment. In the analog dimming mode,the initial light output can be determined by an initial voltage of thereference signal REF. In the burst dimming mode, the initial lightoutput can be determined by an initial duty cycle (e.g., 100%) of thePWM signal PWM1. The reference signal REF and the PWM signal PWM1 can begenerated by the D/A converter 528 according to a counter value of thecounter 1126, in one embodiment. Therefore, the initial voltage of REFand the initial duty cycle of PWM1 can be determined by an initialcounter value (e.g., zero) provided by the counter 1126.

In order to adjust the light output of the LED string 312, the user canapply a first set of operations to the power switch 304. A dimmingrequest signal is generated upon detection of the first set ofoperations of the power switch 304. In one embodiment, the first set ofoperations can include a first turn-off operation followed by a firstturn-on operation. As a result, a dimming request signal including anegative edge 1204 followed by a positive edge 1206 of the voltage atthe terminal CLK can be detected and received by the trigger monitoringunit 1106. In response to the dimming request signal, the triggermonitoring unit 1106 can generate a signal EN having a high level. Thus,the clock generator 1104 is enabled to generate a clock signal. Thecounter 1126 driven by the clock signal can change the counter value inresponse to each clock pulse of the clock signal. In the example of FIG.12, the counter value increases in response to the clock signal. In oneembodiment, the counter value can be reset to zero after the counter1126 reaches its predetermined maximum counter value. In anotherembodiment, the counter value increases until the counter 1126 reachesits predetermined maximum counter value, and then decreases until thecounter 1126 reaches its predetermined minimum counter value.

In the analog dimming mode, the D/A converter 528 reads the countervalue from the counter 1126 and decreases the voltage of the referencesignal REF in response to an increase of the counter value, in oneembodiment. In the burst dimming mode, the D/A converter 528 reads thecounter value from the counter 1126 and decreases the duty cycle of thePWM signal PWM1 (e.g., decreases 10% each time) in response to anincrease of the counter value, in one embodiment. Accordingly, the lightoutput of the LED string 312 can be adjusted because the regulated powerfrom the power converter 310 can be determined by the voltage of thereference signal REF (in the analog dimming mode) or by the duty cycleof the PWM signal PWM1 (in the burst dimming mode).

Once a desired light output has been achieved, the user can terminatethe adjustment process by applying a second set of operations to thepower switch 304. A dimming termination signal is generated upondetection of the second set of operations of the power switch 304. Inone embodiment, the second set of operations can include a secondturn-off operation followed by a second turn-on operation. As a result,the dimming termination signal including a negative edge 1208 followedby a positive edge 1210 of the voltage at the terminal CLK can bedetected and received by the trigger monitoring unit 1106. Upondetection of the dimming termination signal, the trigger monitoring unit1106 can generate the signal EN having a low level. Thus, the clockgenerator 1104 is disabled, such that the counter 1126 can hold itscounter value. Accordingly, in the analog dimming mode, the voltage ofthe reference signal REF can be held at a desired level. In the burstdimming mode, the duty cycle of the PWM signal PWM1 can be held at adesired value. Therefore, the light output of the LED string 312 can bemaintained at a desired light output.

FIG. 13 shows a flowchart 1300 of a method for adjusting power of alight source, in accordance with one embodiment of the presentinvention. FIG. 13 is described in combination with FIG. 10 and FIG. 11.

In block 1302, a light source, e.g., the LED string 312, is powered by aregulated power from a power converter, e.g., the power converter 310.

In block 1304, a dimming request signal can be received, e.g., by thedimming controller 1108. The dimming request signal can indicate a firstset of operations of a power switch, e.g., the power switch 304 coupledbetween a power source and the power converter. In one embodiment, thefirst set of operations of the power switch includes a first turn-offoperation followed by a first turn-on operation.

In block 1306, the regulated power from the power converter iscontinuously adjusted, e.g., by the dimming controller 1108. In oneembodiment, a clock generator 1104 can be enabled to drive a counter1126. A dimming signal (e.g., control signal 538 or reference signalREF) can be generated according to the counter value of the counter1126. In an analog dimming mode, the regulated power from the powerconverter can be adjusted by comparing the reference signal REF with afeedback current monitoring signal which indicates a light sourcecurrent of the light source. The voltage of REF can be determined by thecounter value. In a burst dimming mode, the regulated power from thepower converter can be adjusted by varying a duty cycle of a PWM signalPWM1 by the control signal 538. The duty cycle of PWM1 can be alsodetermined by the counter value.

In block 1308, a dimming termination signal can be received, e.g., bythe dimming controller 1108. The dimming termination signal can indicatea second set of operations of a power switch, e.g., the power switch 304coupled between a power source and the power converter. In oneembodiment, the second set of operations of the power switch includes asecond turn-off operation followed by a second turn-on operation.

In block 1310, the adjustment of the regulated power from the powerconverter is terminated if the dimming termination signal is received.In one embodiment, the clock generator 1104 is disabled such that thecounter 1126 can hold its counter value. As a result, in the analogdimming mode, the voltage of REF can be held at a desired level. In theburst dimming mode, the duty cycle of the PWM signal PWM1 can be held ata desired value. Consequently, the light source can maintain a desiredlight output.

FIG. 14A shows an example of a schematic diagram of a light sourcedriving circuit 1400, in accordance with one embodiment of the presentinvention. Elements labeled the same as in FIG. 3 and FIG. 4 havesimilar functions. FIG. 14A is described in combination with FIG. 4. Thelight source driving circuit 1400 is coupled to a power source V_(IN)(e.g., 110/120 Volt AC, 60 Hz) via a power switch 304 and is coupled toan LED light source 312. Referring to FIG. 14B, an example of the powerswitch 304 in FIG. 14A is illustrated according to one embodiment of thepresent invention. In one embodiment, the power switch 304 is an on/offswitch mounted on the wall. By switching an element 1480 to an ON placeor an OFF place, the conductance status of the power switch 304 iscontrolled on or off, e.g., by a user.

Referring back to FIG. 14A, the light source driving circuit 1400includes an AC/DC converter 306, a power converter 310, and a dimmingcontroller 1408. The AC/DC converter 306 converts an input AC voltageV_(IN) to an output DC voltage V_(OUT). In the example of FIG. 14A, theAC/DC converter 306 includes a bridge rectifier including diodes D1, D2,D7 and D8. The power converter 310 coupled to the AC/DC converter 306receives the output DC voltage V_(OUT) and provides output power to theLED light source 312. The dimming controller 1408 coupled to the AC/DCconverter 306 and coupled to the power converter 310 is operable formonitoring the power switch 304, and for regulating the output power ofthe power converter 310 according to operations of the power switch 304so as to control brightness of light emitted from the LED light source312.

In one embodiment, the power converter 310 includes an inductor L1, adiode D4, a switch Q27, a control switch Q16, and a current sensor R5.The dimming controller 1408 includes multiple terminals, such as aterminal HV_GATE, a terminal CLK, a terminal VDD, a terminal GND, aterminal CTRL, a terminal RT and a terminal MON. The terminals of thedimming controller 1408 operate similarly as the corresponding terminalsof the dimming controller 308 described in relation to FIG. 4.

During operation, the dimming controller 1408 monitors the power switch304 by receiving a switch monitoring signal 1450 at the terminal CLK.The switch monitoring signal 1450 indicates a conductance status, e.g.,ON/OFF status, of the power switch 304. Accordingly, the dimmingcontroller 1408 controls the switch Q27 through the terminal HV_GATE andcontrols the control switch Q16 through the terminal CTRL, so as tocontrol the dimming of the LED light source 312.

More specifically, in one embodiment, when the power switch 304 isturned on, the dimming controller 1408 generates a signal, e.g., logichigh, at the terminal HV_GATE to turn the switch Q27 on, and generates aswitch control signal 1452 at the terminal CTRL to turn the controlswitch Q16 on and off. In one embodiment, the control switch Q16operates in a switch-on state and a switch-off state. During theswitch-on state of the control switch Q16, the switch control signal1452 alternately turns the control switch Q16 on and off. For example,the dimming controller 1408 periodically turns on the control switchQ16. In addition, the dimming controller 1408 receives a sensing signal1454 via the terminal MON indicating the current I_(LED) through the LEDlight source 312, and turns off the control switch Q16 if the sensingsignal 1454 indicates that the current I_(LED) reaches a currentthreshold I_(TH). Thus, the current I_(LED) ramps up when the controlswitch Q16 is turned on and ramps down when the control switch Q16 isturned off. In this way, the dimming controller 1408 determines a peaklevel of the current I_(LED), such that an average level I_(AVERAGE) ofthe current I_(LED) is controlled. During the switch-off state of thecontrol switch Q16, the switch control signal 1452 maintains the controlswitch Q16 off to cut off the current I_(LED). In one embodiment, thedimming controller 1408 determines a time ratio of the switch-on stateto the switch-off state to control the average level I_(AVERAGE) of thecurrent I_(LED).

When the power switch 304 is turned off, the dimming controller 1408generates a signal, e.g., logic low, at the terminal HV_GATE to turn offthe switch Q27, in one embodiment. As such, the current I_(LED) flowingthrough the LED light source 312 drops to substantially zero ampere tocut off the LED light source 312.

In one embodiment, the dimming controller 1408 receives the switchmonitoring signal 1450 indicating a conductance status of the powerswitch 304 at the terminal CLK. Accordingly, the dimming controller 1408is able to identify an operation of the power switch 304 and provide adimming request signal indicating the operation of the power switch 304.In one embodiment, the dimming controller 1408 provides a dimmingrequest signal if a turn-off operation of the power switch 304 isidentified. Alternatively, the dimming controller 1408 provides adimming request signal if a turn-on operation of the power switch 304 isidentified. In response, the dimming controller 1408 operates in ananalog dimming mode, a burst dimming mode, or a combination mode toadjust the on/off state of the control switch Q16 to control the dimmingof the LED light source 312, in one embodiment. For example, in theanalog dimming mode, the peak level of the current I_(LED) is determinedby the dimming controller 1408 while the time ratio of the switch-onstate to the switch-off state remains at the same level. In the burstdimming mode, the time ratio of the switch-on state to the switch-offstate is determined by the dimming controller 1408 while the peak levelof the current I_(LED) remains at the same level. In the combinationmode, both the peak level of the current I_(LED) and the time ratio ofthe switch-on state to the switch-off state are determined by thedimming controller 1408. Thus, when the switch Q27 is turned on again(indicating the power switch 304 is turned on again), the peak level ofthe current I_(LED) and/or the time durations of the switch-on state andthe switch-off state are adjusted. As a result, the average currentI_(AVERAGE) flowing through the LED light source 312 is adjusted tocontrol the brightness of the LED light source 312.

Advantageously, by adjusting both the peak level of the current I_(LED)and the durations of the switch-on state and the switch-off state, thedimming controller 1408 is able to adjust the average currentI_(AVERAGE) in a relatively wide range. For example, if I_(MAX) is amaximum level of the average current I_(AVERAGE), I_(AVERAGE) can varyin a range of 4%*I_(MAX) to 100%*I_(MAX) in accordance with oneembodiment, compared to a range of 20%*I_(MAX) to 100%*I_(MAX) inconventional art. Consequently, a wider range dimming for the LED lightsource 312 is achieved, which is beneficial for energy-efficient lightapplications, for example, night lighting.

FIG. 15 shows an example of a structure of the dimming controller 1408in FIG. 14A, in accordance with one embodiment of the present invention.FIG. 15 is described in combination with FIG. 5-FIG. 7 and FIG. 14A.Elements labeled the same as in FIG. 5 and FIG. 14A have similarfunctions.

In the example of FIG. 15, the dimming controller 1408 includes astart-up and UVL circuit 508, a pulse signal generator 504, a triggermonitoring unit 506, a dimmer 1502, a comparator 534, an SR flip-flop522, and an AND gate 524. The dimmer 1502 includes a reference signalgenerator 1506 for generating a reference signal REF and furtherincludes a PWM generator 1508 for generating a pulse-width modulationsignal PWM1. As described in relation to FIG. 5, the comparator 534compares the sensing signal 1454 with the reference signal REF togenerate a comparing signal COMP. The pulse signal generator 504generates a pulse signal 536 having a waveform of periodical pulses. Inone embodiment, the SR flip-flop 522 sets the pulse signal V₅₂₂ todigital one when the pulse signal 536 is digital one and resets thepulse signal V₅₂₂ to digital zero when the comparing signal COMP isdigital one (e.g., when the sensing signal 1454 reaches the referencesignal REF). The AND gate 524 receives the pulse signal V₅₂₂ and thepulse-width modulation signal PWM1, and generates the switch controlsignal 1452 accordingly to control the control switch Q16.

Assuming that the switch Q27 is turned on, the dimming controller 1408controls the current I_(LED) in a similar way as the dimming controller308 described in relation to FIG. 6 and FIG. 7. During a first state ofthe pulse-width modulation signal PWM1 (e.g., PWM1 is digital one), theAND gate 524 alternately turns on and off the control switch Q16according to the pulse signal V₅₂₂, in one embodiment. As such, thecontrol switch Q16 operates in the switch-on state, in which the currentI_(LED) ramps up when the control switch Q16 is turned on and ramps downwhen the control switch Q16 is turned off. The reference signal REFdetermines a peak level of the current I_(LED) by turning off thecontrol switch Q16 when the sensing signal 1454 reaches the referencesignal REF. During a second state of the pulse-width modulation signalPWM1 (e.g., PWM1 is digital zero), the AND gate 524 maintains thecontrol switch Q16 to be off. As such, the control switch Q16 operatesin the switch-off state to cut off the current I_(LED).

Therefore, the reference signal REF is used to determine the peak levelof the current I_(LED), and the duty cycle of the pulse-width modulationsignal PWM1 is used to determine the time ratio of the switch-on stateto the switch-off state of the control switch Q16. In other words, theaverage current I_(AVERAGE) through the LED light source 312 variesaccording to the reference signal REF and the duty cycle of the PWM1.For example, I_(AVERAGE) is increased if the voltage V_(REF) of thereference signal REF is increased and is decreased if V_(REF) isdecreased. Moreover, I_(AVERAGE) is increased if the duty cycle D_(PWM1)of PWM1 is increased and is decreased if D_(PWM1) is decreased.

The dimmer 1502 further includes a counter 1504 for providing a countervalue. In one embodiment, the reference signal generator 1506 coupled tothe counter 1504 determines the voltage level V_(REF) based upon thecounter value VALUE_1504 of the counter 1504. The PWM generator 1508coupled to the counter 1504 determines the duty cycle D_(PWM1) basedupon the counter value VALUE_1504.

TABLE 1 VALUE_1504 0 1 2 3 V_(REF) V_(MAX) 50% * V_(MAX) 20% * V_(MAX)20% * V_(MAX) D_(PWM1) 100% 100% 100% 20%

TABLE 2 VALUE_1504 0 1 2 3 V_(REF) V_(MAX) 50% * V_(MAX) 30% * V_(MAX)20% * V_(MAX) D_(PWM1) 100% 60% 40% 20%

Table 1 and Table 2 show examples of the counter value VALUE_1504 of thecounter 1504 versus the voltage V_(REF) and the duty cycle D_(PWM1). Inone embodiment, the counter 1504 is a 2-bit counter, and thus thecounter value can be 0, 1, 2 or 3. V_(MAX) represents a maximum voltagelevel of the reference signal REF. According to Table 1, when thecounter value VALUE_1504 is 0, 1, 2 and 3, the reference signal REF haslevels V_(MAX), 50%*V_(MAX), 20%*V_(MAX) and 20%*V_(MAX), respectively,and the duty cycle D_(PWM1) has values 100%, 100%, 100% and 20%,respectively. According to Table 2, when the counter value VALUE_1504 is0, 1, 2 and 3, the reference signal REF has levels V_(MAX), 50%*V_(MAX),30%*V_(MAX) and 20%*V_(MAX), respectively, and the duty cycle D_(PWM1)has values 100%, 60%, 40% and 20%, respectively. The counter value, thereference signal REF and the duty cycle of PWM1 can have otherrelationships, and are not limited to the examples in Table 1 and Table2.

If a dimming request signal, e.g., indicating a turn-off operation ofthe power switch 304, is received, the trigger monitoring unit 506generates an enable signal 1510, in one embodiment. The counter 1504receives the enable signal 1510 and increases or decreases the countervalue accordingly. As such, the reference signal generator 1506determines the reference signal REF, e.g., according to Table 1 or Table2. The PWM generator 1508 determines the duty cycle of PWM1, e.g.,according to Table 1 or Table 2.

As a result, the dimming controller 1408 selectively operates in ananalog dimming mode, a burst dimming mode, and a combination mode. Inthe analog dimming mode, the level of the reference signal REF isdetermined by the counter value of the counter 1504 to adjust theaverage current I_(AVERAGE) while the duty cycle D_(PWM1) of PWM1remains at the same level, in one embodiment. In the bust dimming mode,the duty cycle D_(PWM1) of PWM1 is determined by the counter value ofthe counter 1504 to adjust the average current I_(AVERAGE) while thereference signal REF remains at the same level, in one embodiment. Inthe combination mode, both the level of the reference signal REF and theduty cycle D_(PWM1) are determined according to the counter value of thecounter 1504. Therefore, the brightness of the LED light source 302 isadjusted. The operations of the dimming controller 1408 are furtherdescribed in relation to FIG. 16 and FIG. 17. The dimming controller1408 can have other configurations and is not limited to the exampleshown in FIG. 15.

FIG. 16 illustrates an example of a diagram illustrating an operation ofa light source driving circuit which includes the dimming controller1408 in FIG. 15, in accordance with one embodiment of the presentinvention. FIG. 16 is described in combination with FIG. 14A and FIG.15. FIG. 16 shows the voltage V_(CLK) at the terminal CLK, the countervalue VALUE_1504 of the counter 1504, the voltage V_(PWM1) of thepulse-width modulation signal PWM1, the duty cycle D_(PWM1) of thepulse-width modulation signal PWM1, the voltage V_(REF) of the referencesignal REF, the voltage V_(SENSE) of the sensing signal 1454, and theaverage level I_(AVERAGE) of the current I_(LED). In the example of FIG.16, the dimming controller 1408 sets the voltage V_(REF) and the dutycycle D_(PWM1) according to the example presented in Table 1.

At time t0, the power switch 304 is off. The counter value VALUE_1504 is0. Based upon Table 1, the duty cycle D_(PWM1) is 100% and the voltageV_(REF) has the maximum level V_(MAX). Since the power switch 304 andthe switch Q27 are both turned off, the current I_(LED) is cut off andthus the average current I_(AVERAGE) is zero.

At time t1, the voltage V_(CLK) has a rising edge indicating a turn-onoperation of the power switch 304. The dimming controller 1408 turns onthe switch Q27, and thus the current I_(LED) is controlled according tothe conductance status of the control switch Q16. Between t1 and t2, theduty cycle D_(PWM1) is 100% and the voltage V_(REF) has the maximumlevel V_(MAX). The control switch Q16 operates in the switch-on state tobe alternately on and off. As shown in FIG. 16, the voltage V_(SENSE)ramps up when the control switch Q16 is on and ramps down when thecontrol switch Q16 is off. Since the peak level of the voltage V_(SENSE)is equal to the maximum level V_(MAX) of the reference signal REF, theaverage current I_(AVERAGE) has a maximum level I_(MAX).

At time t2, the voltage V_(CLK) has a falling edge indicating a turn-offoperation of the power switch 304. The switch Q27 is turned off to cutoff the current I_(LED). Thus, between t2 and t3, the voltage V_(SENSE)drops to substantially zero volt and the average current I_(AVERAGE)drops to substantially zero ampere.

In one embodiment, upon detection of a turn-off operation of the powerswitch 304 at time t2, a dimming request signal is generated. Thecounter value VALUE_1504 is increased from 0 to 1. Based upon theexample in Table 1, the dimming controller 1408 is switched to an analogdimming mode to adjust the voltage V_(REF) to 50%*V_(MAX) and maintainsthe duty cycle D_(PWM1) at 100%.

At time t3, the switch Q27 is turned on again. Thus, during the timeinterval between t3 and t4, the dimming controller 1408 switches thecontrol switch Q16 on and off according to the reference signal REF andthe pulse-width modulation signal PWM1. Thus, the average currentI_(AVERAGE) is adjusted to 50%*I_(MAX).

At time t4, the voltage V_(CLK) has a falling edge indicating a turn-offoperation of the power switch 304. The counter value VALUE_1504 isincreased from 1 to 2. Based upon Table 1, the dimming controller 1408is in the analog dimming mode to adjust the voltage V_(REF) to20%*V_(MAX) and maintain the duty cycle D_(PWM1) at 100%. Thus, theaverage current I_(AVERAGE) is adjusted to 20%*I_(MAX) between t5 andt6.

At time t6, a falling edge of the voltage V_(CLK) indicates a turn-offoperation of the power switch 304. In response, the counter value isincreased from 2 to 3. Based upon Table 1, the dimming controller 1408is switched to a burst dimming mode to maintain the voltage V_(REF) at20%*V_(MAX) and decrease the duty cycle D_(PWM1) to 20%. As such, whenthe power switch 304 is turned on during a time interval between t7 andt8, the voltage V_(SENSE) ramps up and down when the voltage V_(PWM1)has a first state, e.g., logic high, and drops to substantially zerovolt when the voltage V_(PWM1) has a second state, e.g., logic low. Assuch, the average current I_(AVERAGE) is adjusted to 4%*I_(MAX) betweent7 and t8.

Therefore, in the example of FIG. 16, the dimming controller 1408initially operates in the analog dimming mode to adjust the averagecurrent I_(AVERAGE) from 100%*I_(MAX) to 20%*I_(MAX) and then operatesin the burst dimming mode to adjust the average current I_(AVERAGE) from20%*I_(MAX) to 4%*I_(MAX). Advantageously, both the duty cycle D_(PWM1)and the voltage V_(REF) are adjusted to achieve the average currentI_(AVERAGE) in a range of 100%*I_(MAX) to 4%*I_(MAX). Thus, the dimmingof the LED light source 312 is achieved in a wider range. Moreover,during the relatively wide range of dimming, the voltage V_(REF) ismaintained greater than a voltage threshold (e.g., 15%*V_(MAX)) and theduty cycle D_(PWM1) is maintained greater than a duty cycle threshold(e.g., 10%). As such, the accuracy of the reference signal REF and thepulse-width modulation signal PWM1 is not affected by undesirableconditions such as noises, which improves the dimming accuracy of thelight source driving circuit 1400.

FIG. 17 illustrates another example of a diagram illustrating anoperation of a light source driving circuit which includes the dimmingcontroller 1408 in FIG. 15, in accordance with one embodiment of thepresent invention. FIG. 17 is described in combination with FIG.14A-FIG. 16. FIG. 17 shows the voltage V_(CLK) at the terminal CLK, thecounter value VALUE_1504 of the counter 1504, the voltage V_(PWM1) ofthe pulse-width modulation signal PWM1, the duty cycle D_(PWM1) of thepulse-width modulation signal PWM1, the voltage V_(REF) of the referencesignal REF, the voltage V_(SENSE) of the sensing signal 1454, and theaverage level I_(AVERAGE) of the current I_(LED). In the example of FIG.17, the dimming controller 1408 sets the voltage V_(REF) and the dutycycle D_(PWM1) according to the example presented in Table 2.

Between t0′ and t2′, the dimming controller 1408 operates similarly tothe operation between t0 and t2 as described in relation to FIG. 16. Forexample, the counter value VALUE_1504 is 0 between t0′ and t2′. Basedupon Table 2, the duty cycle D_(PWM1) is 100% and the voltage V_(REF)has the maximum level V_(MAX). Thus, between t1′ and t2′, the peak levelof the voltage V_(SENSE) is equal to the maximum level V_(MAX) of thereference signal REF and the average current I_(AVERAGE) has a maximumlevel I_(MAX).

At time t2′, a falling edge of the voltage V_(CLK) indicates a turn-offoperation of the power switch 304. The switch Q27 is turned off to cutoff the current I_(LED). Thus, between t2′ and t3′, the voltageV_(SENSE) drops to substantially zero volt and the average currentI_(AVERAGE) drops to substantially zero ampere.

In one embodiment, upon detection the turn-off operation of the powerswitch 304 at time t2′, a dimming request signal is generated. Thecounter value VALUE_1504 is increased from 0 to 1. Based upon theexample in Table 2, the dimming controller 1408 operates in thecombination mode to adjust the voltage V_(REF) to 50%*V_(MAX) and adjustthe duty cycle D_(PWM1) to 60%. Therefore, between t3′ and t4′, thecontrol switch Q16 operates in the switch-on state to alternately on andoff according to the pulse signal V₅₂₂ when the voltage V_(PWM1) has afirst state, e.g., logic high. The peak level of the voltage V_(SENSE)is equal to the voltage V_(REF), that is, 50%*V_(MAX). Moreover, thecontrol switch Q16 operates in the switch-off state to cut off thecurrent I_(LED) when the voltage V_(PWM1) has a second state, e.g.,logic low. Thus, the average level of the current I_(LED) is equal to30%*I_(MAX).

At time t4′, a falling edge of the voltage V_(CLK) indicates a turn-offoperation of the power switch 304, and thus a dimming request signal isgenerated. In response, the counter value VALUE_1504 is increased from 1to 2. Based upon the example in Table 2, the dimming controller 1408operates in the combination mode to adjust the voltage V_(REF) to30%*V_(MAX) and adjust the duty cycle D_(PWM1) to 40%. Consequently, theaverage level of the current I_(LED) is equal to 12%*I_(MAX) between t5′and t6′.

At time t6′, a falling edge of the voltage V_(CLK) indicates a turn-offoperation of the power switch 304 and thus a dimming request signal isgenerated. In response, the counter value VALUE_1504 is increased from 2to 3. Based upon the example in Table 2, the dimming controller 1408operates in the combination mode to adjust the voltage V_(REF) to20%*V_(MAX) and adjust the duty cycle D_(PWM1) to 20%. Consequently, theaverage level of the current I_(LED) is equal to 4%*I_(MAX) between t7′and t8′.

Therefore, between t1′ and t7′, the dimming controller operates in thecombination mode when the counter value VALUE_1504 is changed.Advantageously, both the duty cycle D_(PWM1) and the voltage V_(REF) areadjusted to achieve the average current I_(AVERAGE) in a range of100%*I_(MAX) to 4%*I_(MAX). The dimming of the LED light source 302 areachieved in a wider dimming range. Moreover, during the relatively widerange of dimming, the voltage V_(REF) is maintained greater than avoltage threshold (e.g., 15%*V_(MAX)) and the duty cycle D_(PWM1) ismaintained greater than a duty cycle threshold (e.g., 10%). As such, theaccuracy of the reference signal REF and the pulse-width modulationsignal PWM1 is not affected by undesirable conditions such as noises,which improves the dimming accuracy of the light source driving circuit1400.

FIG. 18 shows a flowchart 1800 of a method for controlling dimming of anLED light source, in accordance with one embodiment of the presentinvention. FIG. 18 is described in combination with FIG. 14A-FIG. 17.Although specific steps are disclosed in FIG. 18, such steps areexamples. That is, the present invention is well suited to performingvarious other steps or variations of the steps recited in FIG. 18.

In block 1802, a sensing signal, e.g., the sensing signal 1454,indicative of a current flowing through the LED light source, e.g.,I_(LED), is compared to a reference signal, e.g., the reference signalREF, to provide a pulse signal, e.g., the pulse signal V₅₂₂. In block1804, the current through the LED light source is controlled accordingto the pulse signal during a first state of a pulse-width modulationsignal, e.g., PWM1. In block 1806, the current through the LED lightsource is cut off during a second state of a pulse-width modulationsignal.

In block 1808, both a level of the reference signal and the duty cycleof the pulse-width modulation signal are adjusted based upon a dimmingrequest signal. In one embodiment, a counter value of a counter isadjusted according to the dimming request signal. The level of thereference signal and the duty cycle of the pulse-width modulation signalare determined according to the counter value. If the counter value ischanged from a first value to a second value, a first mode (e.g., ananalog dimming mode), a second mode (e.g., a burst dimming mode), or athird mode (e.g., a combination mode) is selected. In the first mode,the level of the reference signal is adjusted and the duty cycle of thepulse-width modulation signal is maintained. In the second mode, thelevel of the reference signal is maintained and the duty cycle of thepulse-width modulation signal is adjusted. In the third mode, both thelevel of the reference signal and the duty cycle of the pulse-widthmodulation signal are adjusted.

Accordingly, embodiments in accordance with the present inventionprovide controllers, circuits and methods for driving an LED lightsource including LEDs. A light source driving circuit includes acontroller for providing a pulse signal by comparing a sensing signalindicative of a current flowing through the LED light source to areference signal, and for controlling the current through said LED lightsource according to the pulse signal during a first state of apulse-width modulation signal and cutting off the current through theLED light source during a second state of the pulse-width modulationsignal. Advantageously, the controller adjusts both the level of areference signal and the duty cycle of the pulse-width modulation signalto achieve dimming of the LED light source in a wider range. Moreover,the level of the reference signal is maintained greater than a voltagethreshold and the duty cycle of the pulse-width modulation signal ismaintained greater than a duty cycle threshold. Thus, the dimmingaccuracy of the light source driving circuit is improved.

While the foregoing description and drawings represent embodiments ofthe present invention, it will be understood that various additions,modifications and substitutions may be made therein without departingfrom the spirit and scope of the principles of the present invention asdefined in the accompanying claims. One skilled in the art willappreciate that the invention may be used with many modifications ofform, structure, arrangement, proportions, materials, elements, andcomponents and otherwise, used in the practice of the invention, whichare particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims and theirlegal equivalents, and not limited to the foregoing description.

1. A controller controlling dimming of a light-emitting diode (LED)light source, said controller providing a pulse signal by comparing asensing signal indicative of a current flowing through said LED lightsource to a reference signal, and said controller controlling saidcurrent through said LED light source according to said pulse signalduring a first state of a pulse-width modulation signal and cutting offsaid current through said LED light source during a second state of saidpulse-width modulation signal, said controller receiving a dimmingrequest signal indicative of an operation of a power switch coupledbetween an AC power source and a bridge rectifier and controlling both alevel of said reference signal and a duty cycle of said pulse-widthmodulation signal based on said dimming request signal.
 2. Thecontroller as claimed in claim 1, further comprising: a counterproviding a counter value that varies according to said dimming requestsignal, wherein said controller maintains the level of said referencesignal and adjusts the duty cycle of said pulse-width modulation signalif said counter value is changed from a first value to a second value.3. The controller as claimed in claim 1, further comprising: a counterproviding a counter value that varies according to said dimming requestsignal, wherein said controller adjusts the level of said referencesignal and maintains the duty cycle of said pulse-width modulationsignal if said counter value is changed from a first value to a secondvalue.
 4. The controller as claimed in claim 1, further comprising: acounter providing a counter value that varies according to said dimmingrequest signal, wherein said controller adjusts both the level of saidreference signal and the duty cycle of said pulse-width modulationsignal if said counter value is changed from a first value to a secondvalue.
 5. The controller as claimed in claim 1, further comprising: atrigger monitoring unit receiving a monitoring signal indicative of aconductance status of said power switch, and identifying said operationof said power switch according to said monitoring signal.
 6. Thecontroller as claimed in claim 1, further comprising: control circuitryalternately turning on and off a control switch in series with said LEDlight source according to said pulse signal during said first state ofsaid pulse-width modulation signal and maintaining said control switchoff during said second state of said pulse-width modulation signal. 7.The controller as claimed in claim 1, further comprising: controlcircuitry switching said pulse signal to a first state according to saidcomparison of said sensing signal to said reference signal and switchingsaid pulse signal to a second state.
 8. A driving circuit for powering alight-emitting diode (LED) light source, said driving circuitcomprising: a control switch coupled to said LED light source in series;and a controller coupled to said control switch, providing a pulsesignal by comparing a sensing signal indicative of a current flowingthrough said LED light source to a reference signal, switching saidcontrol switch on and off alternately according to said pulse signalduring a first state of a pulse-width modulation signal, maintainingsaid control switch off during a second state of said pulse-widthmodulation signal, and adjusting both a level of said reference signaland a duty cycle of said pulse-width modulation signal based on adimming request signal indicative of an operation of a power switchcoupled between an AC power source and a bridge rectifier.
 9. Thedriving circuit as claimed in claim 8, wherein said controller furthercomprises: a dimmer adjusting a counter value of a counter according tosaid dimming request signal, and determining the level of said referencesignal and the duty cycle of said pulse-width modulation signalaccording to said counter value.
 10. The driving circuit as claimed inclaim 9, wherein said dimmer changes the level of said reference signaland maintains the duty cycle of said pulse-width modulation signal ifsaid counter value is adjusted from a first value to a second value. 11.The driving circuit as claimed in claim 9, wherein said dimmer maintainsthe level of said reference signal and changes the duty cycle of saidpulse-width modulation signal if said counter value is adjusted from afirst value to a second value.
 12. The driving circuit as claimed inclaim 9, wherein said dimmer changes both the level of said referencesignal and the duty cycle of said pulse-width modulation signal if saidcounter value is adjusted from a first value to a second value.
 13. Thedriving circuit as claimed in claim 8, further comprising: a powerconverter receiving power from said bridge rectifier and providingregulated power to said LED light source, wherein said controlleridentifies said operation of said power switch, and wherein saidoperation of said power switch is selected from a turn-on operation anda turn-off operation.
 14. The driving circuit as claimed in claim 8,further comprising: a third switch coupled in series with said LED lightsource, wherein said controller switches said third switch on and offaccording to a conductance status of said power switch.
 15. A controllercontrolling dimming of a light-emitting diode (LED) light source, saidcontroller selectively operating in at least a first mode and a secondmode, wherein said controller provides a pulse signal by comparing asensing signal to a reference signal, controls a current flowing throughsaid LED light source according to said pulse signal during a firststate of a pulse-width modulation signal, and cuts off said currentthrough said LED light source during a second state of said pulse-widthmodulation signal, wherein in said first mode said controller adjusts anaverage current through said LED light source by adjusting a level ofsaid reference signal, and wherein in said second mode said controlleradjusts said average current through said LED light source by adjustinga duty cycle of said pulse-width modulation signal.
 16. The controlleras claimed in claim 15, wherein said controller further operates in athird mode in which said controller adjusts said average current throughsaid LED light source by adjusting both the level of said referencesignal and the duty cycle of said pulse-width modulation signal.
 17. Thecontroller as claimed in claim 15, further comprising: a dimmeradjusting a counter value of a counter according to a dimming requestsignal indicative of an operation of a power switch coupled between anAC power source and a bridge rectifier and determining the level of saidreference signal and the duty cycle of said pulse-width modulationsignal according to said counter value.
 18. The controller as claimed inclaim 17, wherein said controller is switched between said first modeand said second mode according to a variation of said counter value. 19.The controller as claimed in claim 17, further comprising: a triggermonitoring unit coupled to said dimmer, monitoring said power switch,and generating said dimming request signal indicating a turn-offoperation of said power switch.
 20. The controller as claimed in claim15, further comprising: control circuitry switching said pulse signal toa first state according to said comparison of said sensing signal tosaid reference signal and switching said pulse signal to a second state.